Genome-wide Identification and Characterization of the Ascorbate Peroxidase Gene Family in Citrus sinensis in Response to Huanglongbing

Ascorbate peroxidases (APXs) are essential for plants as they act as hydrogen peroxide-scavenging enzymes, providing protection against oxidative damage. Using bioinformatic methods, five APX genes were discovered in the genome of Citrus sinensis in this study. APX genes of C. sinensis (CsAPXs) encode polypeptides between 250 and 436 residues in length, with molecular weights that range from 27.56 to 47.34 kDa. Additionally, the isoelectric point of CsAPXs varies from 5.64 to 8.63. The predicted locations of CsAPXs are peroxisome, chloroplast, and mitochondrion, with an uneven distribution across four chromosomes and eight orthologous gene pairs with Arabidopsis thaliana. A phylogenetic analysis revealed that the CsAPXs were divided into three clades. The CsAPXs all contained a conserved APX domain and six common motifs. Upon promoter analysis, it was found that CsAPXs could respond to abscisic acid, auxin, ethylene, gibberellin, methyl jasmonate, salicylic acid, and wounding stress treatments. In addition, the analysis of expression patterns revealed that the presence of Candidatus Liberibacter asiaticus (CLas) has a dynamic impact on the expression of CsAPXs, with CsAPX2 showing significant inhibition in response to CLas infection. These findings will provide novel insights for the forthcoming functional investigations of CsAPXs within the process of citrus-CLas interactions.